Creating micro-scale surface textures on the exterior of interface parts has potentially promising energy and biomedical applications wherein creation of new energy resource and saving of energy consumption can be enabled through innovative micro-forming technologies. The micro-manufacturing technology which leads to reduction of energy consumption and environmental pollutions will bridge macro-scale production and nano-scale enabling science to make it possible for realizing innovative ideas on energy sustainability. For example, arrayed microchannel device (AMD) is a stack of laminas contacted through channel heads. Each lamina consists of hundreds of microchannels that facilitate heat exchange or mass transfer. AMD has a direct impact on increasing energy efficiency in the application areas, such as distributed power generation, hydrogen generation and fuel cells. Micro-channel heat exchangers have demonstrated heat fluxes 3 to 5 times higher than conventional heat exchangers [reference 1]. As energy efficiency has become increasingly important due to escalating fuel costs, the desire to reduce friction and wear of contacting surfaces has intensified. It is estimated that up to one third of all energy usage worldwide is used to overcome friction, resulting in a potentially significant energy savings with even moderate friction reduction [reference 2]. Surface texturing is a nontraditional technique for friction reduction; rather than maintaining a very smooth surface, dimples are intentionally created on the surface of a part in sliding contact, resulting in a significant friction reduction [references 3-7]. These dimples serve as micro-reservoirs for the lubricant, resulting in a reduction in lubricant leakage. During sliding, lubricant pressure builds up in the dimples, which in turn helps to create a separation between the contact parts. The dimples also function as receptacles for debris and wear particles, eliminating potential scratching of the substrate surface during the relative motion of the interface parts.
Economically sustainable manufacturing methods are needed to enable the commercialization of these applications. Established fabrication methods for micro channels and micro dimples include laser ablation, micro machining, and photochemical machining. Laser ablation utilizes high-intensity laser pulses to incrementally ablate minute segments of the substrate material to create the desired feature geometry [reference 3]. Laser ablation can easily texture nearly any material. However, it requires expensive equipment and surface finish of product is generally poor if a fast process is desired. Micro machining removes material from a substrate using cutting or milling to leave a desired geometry. Micro incremental forming [reference 8], one type of die-less micro forming, forms micro channels of various profiles by depressing and drawing the single point tool along the thin metal sheet. These three methods allow for accurate shapes and tolerances, but are fairly time consuming and expensive. They are only suitable for fabricate prototypes.
Currently, mass production of micro channels and dimples are enabled by photochemical machining, which places an etching mask on the top of specimen to selectively etch the material via etching chemicals [reference 9]. Photochemical machining is able to create almost any geometry that mimics a pre-made pattern. However, the fabrication cost of this process is still too high when high-volume of shims is needed. Besides, photochemical machining also poses material waste and environmental concerns. Most of the chemicals such as cleaning solutions, etchants, strippers etc. are hazardous liquids. Therefore, handling and disposal of them are very costly. The ideal manufacturing must be able to minimize the impact to the environment. Therefore, a method which is capable of efficiently and economically fabricating micro channels and micro dimples is needed.
If the deformation-based micro surface texturing process successfully meets the tolerance requirement, it will lead to many launches of applications in the areas of energy generation, energy utilization and optical illusions. However, during the forming process, the bump or pile-up is built up around the dimples. Futamura et al. [reference 10] developed a micro dimple forming process to improve the anti-seizing properties of sliding surfaces of mechanical components and reduce frictional resistance. They found bumps building up when forming the dimples on A2017 aluminum alloy and S45C carbon steel pipes. After burnishing the bumps, the depth of the dimples on A2017 pile almost kept the same, while the dimples on S45C pipe disappeared totally. The reason for this, difference is that to remove the bump of carbon steel which has a much higher Young's modules and strength than those of the aluminum alloy, burnishing roll has to go deeper, which flattened not only the bumps but also the dimples. Cao et al. [reference 11] investigated the effect of relative velocity on shape distribution of the micro dimples as the relative velocity between the forming tool and sample affects frictional force. They found a clear relationship between relative velocity and frictional force, and its significant effect of relative velocity on the final profile of the dimple.
An economically sustainable micro-manufacturing method embodying deformation-based micro texturing would be beneficial and would enable the commercialization of the above-described applications.